1. Field of the Invention
The present invention relates to an X-ray generator configured such that the advance of electrons generated by a cathode is controlled by a Wehnelt electrode.
2. Description of the Related Art
Typically, in an X-ray generator, electrons generated by a cathode are caused to collide with an anti-cathode. The region of collision of the electrons with the anti-cathode serves as an X-ray focus. X-rays are then generated from this X-ray focus. Techniques for disposing a Wehnelt electrode on the path of advance of the electrons in such an X-ray generator, and controlling the direction of advance of the electrons by the Wehnelt electrode, are known (see Patent Citation 1, for example).
As shown in FIG. 16 which accompanies the description, the X-ray generator shown in Patent Citation 1 has a first Wehnelt electrode 101 provided with a large opening area, and a second Wehnelt electrode 102 provided with a small opening area. A cathode (filament) 103 is disposed within the aperture of the second Wehnelt electrode 102. A voltage V1 is applied across an anti-cathode (target) 104 and the cathode 103. A voltage V2 is applied across the Wehnelt electrodes 101, 102 and the cathode 103.
When the cathode 103 is energized, the cathode 103 radiates heat, generating thermal electrons E. The thermal electrons E, with the direction of advance thereof being controlled by the electrical field formed by the first Wehnelt electrode 101 and the second Wehnelt electrode 102, are accelerated by the voltage V1 and collide with the anti-cathode 104. The region in which the electrons collide is the X-ray focus F, and X-rays are radiated from this X-ray focus F.
In the afore-described conventional X-ray generator, a coil-shaped tungsten filament is employed as the cathode. The first Wehnelt electrode 101 and the second Wehnelt electrode 102 are constituted as an integrated component of a single electrode member. Because the cathode deteriorates with continuous use, it is replaced as needed. During replacement, with the Wehnelt electrodes 101, 102 still disposed at their predetermined positions within the X-ray generator in FIG. 16, the cathode 103 is detached from the second Wehnelt electrode 102, and thereafter the new cathode 103 is attached inside the second Wehnelt electrode 102.
In a case in which the cathode 103 is of a large size, and moreover the accuracy of positioning of the cathode 103 with respect to the second Wehnelt electrode 102 is not so exact, the conventional replacement method can be implemented without difficulty. However, more recently, smaller scale and high brilliance have come to be required of electron sources, which have led to the need for the cathode in such electron sources to be formed to a smaller scale, and for a high degree of accuracy to be stipulated in positioning of the cathode with respect to the second Wehnelt electrode.
In such cases, when the conventional replacement method, specifically, the method whereby, with the Wehnelt electrodes 101, 102 still disposed at their predetermined positions within the X-ray generator, the cathode 103 is detached from the second Wehnelt electrode 102, and thereafter the new cathode 103 is attached inside the second Wehnelt electrode 102, is adopted, it is impossible to position the cathode with the desired positional accuracy with respect to the Wehnelt electrodes 101, 102.